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Electronics => Projects, Designs, and Technical Stuff => Topic started by: johofz on April 22, 2021, 09:41:17 am

Title: Benefits of side plating for sensitive circuits
Post by: johofz on April 22, 2021, 09:41:17 am
Hello dear EEVblog-Community,

I'm currently designing a prototype for a charge mode amplifier used to convert the charge coming from a piezoelectric force/torque sensor.

Since charge mode amps are very sensitive, I thought about applying side plating to the PCB, but I'm not sure if it's worth the extra cost considering it's only a prototype. It would essentially cost almost 3x as much.

I'm dealing with a sensitivity of 3.7pC / N and designed the amp to produce 1V at 10kN. Since it's the first charge mode amp I designed, I'm not 100% sure but It does not seem like a very critical situation in therms of SNR.

One more thing that is typically suggested is to use guard-rings on the inputs of the opamp. Again I have no previous experience whether it is necessary or not in my case.

Any advice would be greatly appreciated!

Greetings
johofz
Title: Re: Benefits of side plating for sensitive circuits
Post by: T3sl4co1l on April 22, 2021, 10:31:48 am
A ground plane and via stitching (with a via fence if you're feeling really paranoid) is more than adequate.

Tim
Title: Re: Benefits of side plating for sensitive circuits
Post by: RoGeorge on April 22, 2021, 10:54:45 am
- Guard rings for op-amps, yes, preferably servo driven so the Vinput to Vring is always zero.
- Side plating, IDK, probably not.
- You'll need to shield the prototype board anyway, sometimes double shielding it, and sometimes magnetically shielding it, too, depending on the working environment.
Title: Re: Benefits of side plating for sensitive circuits
Post by: johofz on April 22, 2021, 11:25:57 am
I decided to use the via fence and a guadr ring around my critical input nodes. I connected the Ring to the reference voltage. is that correct?
Also i read somewhere, that i have to place the guard on the bottem layer aswell if I have through hole components like the test points.

Here is the 3D View of the layout:
[attach=1]

The other inputs of the opamp are for buffering and an preamp to an adc. Should I guard them too? I nevr did sofar and no problems occured. I'm just paranoid since i never dealt with charge amps before.
Title: Re: Benefits of side plating for sensitive circuits
Post by: coppercone2 on April 23, 2021, 05:45:44 pm
if you have trouble you can try to dead bug solder it with through hole components where possible and compare your results to the PCB, it may come out cheaper then prototype iterations

I had great results with dead bug soldering a LMC6001 or whatever that electrometer op amp is, you only have R&C circuits so you are set, and even better if you can get a through hole version of the chip

it should offer the maximum possible leakage performance, those datasheets for electrometer parts often like if you float things above the PCB if possible for you